During the construction of oil and gas wells a bore is drilled into the ground. Casing is then lowered into the bore and the annular space between the casing and the bore filled with cement.
It is important that the casing is held substantially centrally in the bore while the annular space is filled with cement and for this purpose centralisers are mounted on the casing as it is lowered into the bore.
Usually wellbores extend substantially vertically downwardly and the centralisers comprise two spaced apart bands which can be slipped over the casing and a plurality of spring bows which extend between and are connected to the bands.
If part of the wellbore collapses and it is necessary to withdraw the casing then the spring bows simply compress against the side of the casing as they pass through the obstruction after which they expand against the side of the bore.
More recently the demand for deviated drilling has increased significantly. In deviated drilling a branch bore, which can be nearly horizontal, is drilled outwardly from an existing vertical bore.
Many problems arise with deviated drilling. One of these problems is that centralisers with spring bows are not entirely satisfactory since the spring bows will not withstand the lateral forces as the casing turns into the branch bore. Furthermore, the spring bows are generally not strong enough to centre casing in nearly horizontal bores.
One solution to this problem is to use centralisers with rigid members instead of spring bows. Examples of such centralisers are shown in GB-A-2 171 436 and GB-A-682 362. The overall diameter of such centralisers is somewhat smaller than the bore so that said centralisers can be readily moved along the bore. Whilst such centralisers are not as versatile as centralisers with spring bows they do provide a reasonable centralising action.
The disadvantage of such centralisers is that if part of the bore collapses and it is necessary to withdraw the casing some of the rigid members are nearly always broken off as the casing is withdrawn. This is extremely undesirable since the broken fragments can later be carried to the surface and obstruct or damage flow control equipment.